1. Industrial Field of the Invention
The present invention relates to an active matrix type liquid crystal display and to a manufacturing process of a thin film transistor (hereinafter referred to as TFT) used as a drive element in the liquid crystal display.
2. Prior Art
Recent years, the active matrix type liquid crystal display device comprising a TFT array substrate composed by arranging a TFT on a transparent insulating substrate such as glass forming a matrix and a liquid crystal, has been commercialized into a flat display under the expectation of flattening of image display, and has now a bright future in view of developing a large market of notebook type personal computer, OA monitor, etc.
In the greater part of the TFT, an amorphous silicon capable of being deposited on a large area at a relatively low temperature is employed as a semiconductor layer. An example is illustrated in FIG. 3 showing a sectional view of an essential part of a TFT array on which a TFT is mounted and in FIG. 4 showing a sectional view of the TFT array under manufacture. In the drawings, reference numeral 1 designates an insulating substrate, numeral 2 designates a gate electrode formed on the insulating substrate 1, numeral 3 is a gate insulating film formed to coat the gate electrode 2 and an auxiliary capacity electrode 8, numeral 4 is a semiconductor layer composed of a-Si:H (amorphous silicon to which hydrogen atom is added) film 4a formed on the gate electrode 2 through the gate insulating film 3, numeral 5 is an ohmic contact layer composed of a n.sup.+ a-Si:H film 5a formed on the semiconductor layer 4, numeral 9 is a picture element electrode, numerals 10 and 11 are a pair of electrodes (source electrode 10 and drain electrode 11) formed on the ohmic contact layer 5, and numeral 12 is a passivation film formed to coat the entire device.
A manufacturing process of the mentioned conventional TFT is hereinafter described. After forming a first conductive thin film on the insulating substrate 1, the first conductive thin film is patterned by photoengraving process, whereby the gate electrode 2 and the auxiliary capacity electrode 8 are formed. Then, after forming continuously the gate insulating film 3, the a-Si:H film 4a and the n.sup.+ a-Si:H film 5a by plasma CVD, the a-Si:H film 4a and the n.sup.+ a-Si:H film 5a are patterned into an island, whereby the semiconductor layer 4 and the ohmic contact layer 5 are formed. After forming a second conductive thin film, the second conductive thin film is patterned by photoengraving process, whereby the picture element electrode 9 is formed. After forming a third conductive thin film composed of Al--Si alloy, etc., the third conductive thin film is patterned by photoengraving process, whereby the source electrode 10 and the drain electrode 11 are formed. The ohmic contact layer 5 is then etched using the source electrode 10 and the drain electrode 11 as a mask, and the ohmic contact layer 5 is cut into two parts. The passivation film 12 is then formed by plasma CVD. Thus, a TFT array on which TFT is mounted is formed.
FIG. 4 shows a sectional view after having continuously formed the a-Si:H film 4a which forms the semiconductor layer 4 and the n.sup.+ a-Si:H film 5a which forms the ohmic contact layer 5. In the conventional manufacturing process, however, since the a-Si:H film 4a and the n.sup.+ a-Si:H film 5a are further patterned into an island, a photoresist is applied to the two films after performing a wet cleaning, and a resist pattern is formed through the steps of exposure and development. At this time, as the surface of the n.sup.+ a-Si:H film 5a has a hydrophobic property and a low wettability immediately after forming the film, a dry spot is produced at the time of drying after the wet cleaning. When such a dry spot is produced, in the step of etching the ohmic contact layer 5 (the n.sup.+ a-Si:H film 5a) using the source electrode 10 and the drain electrode 11 as a mask, the dry spot performs as the mask and an etch residue is produced or left, which brings about a failure or defect such as short circuit between the source electrode 10 and the drain electrode 11, and if using such a defective TFT as a drive element of a liquid crystal display, any display failure is caused in the liquid crystal display.
To prevent such a dry spot, it is effective to make hydrophilic the surface of the n.sup.+ a-Si:H film 5a which forms the ohmic contact layer 5, and therefore in the conventional manufacturing process, for example, an UV treatment step is added between the step of forming the film by the plasma CVD apparatus and the step of wet cleaning, so that the surface of the n.sup.+ a-Si:H film 5a is oxidized and a hydrophilic property is given thereto.
Since the conventional treatment for giving a hydrophilic property to the surface of the n.sup.+ a-Si:H film 5a, which is applied in view of preventing the production or occurrence of the dry spot resulting in the undesirable etch residue at the time of patterning the n.sup.+ a-Si:H film 5a which forms the ohmic contact layer 5, is an UV treatment as mentioned above, such a treatment cannot be applied by any manufacturing apparatus used just for forming the ohmic contact layer, but any additional treatment step by using other apparatus is required. Hence, there arises a problem of lowering a productivity.